Merge bitcoin/bitcoin#32220: cmake: Get rid of undocumented BITCOIN_GENBUILD_NO_GIT environment variable

3142e5f8cf doc: Add release notes for #32220 (Hennadii Stepanov)
b71cd5c162 cmake: Skip using git when building from source tarball or as subproject (Hennadii Stepanov)
fe941938e8 cmake: Remove unnecessary `BITCOIN_GENBUILD_NO_GIT` environment variable (Hennadii Stepanov)
9a2cced23a cmake, refactor: Move `find_package(Git)` to `src/CMakeLists.txt` (Hennadii Stepanov)

Pull request description:

  In general, the Bitcoin Core build system attempts to fetch commit or tag details from git. This is handled by the [`cmake/script/GenerateBuildInfo.cmake`](https://github.com/bitcoin/bitcoin/blob/master/cmake/script/GenerateBuildInfo.cmake) script, which generates the [`src/bitcoin-build-info.h`](65dcbec756/src/clientversion.cpp (L26-L31)) header within the build tree.

  However, there are cases where the retrieved details may be incorrect—for example, when building from a source tarball or as a subproject within a git-aware project.

  In the Autotools-based build system, the `BITCOIN_GENBUILD_NO_GIT` environment variable was [introduced](https://github.com/bitcoin/bitcoin/pull/7522) in [v0.20.0](https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.20.0.md) to address such scenarios:
  > The process for generating the source code release ("tarball") has changed in an effort to make it more complete, however, there are a few regressions in this release:
  > - Instead of running `make` simply, you should instead run `BITCOIN_GENBUILD_NO_GIT=1 make`.

  This PR automagically handles both of the aforementioned cases and removes the need for `BITCOIN_GENBUILD_NO_GIT`.

  The user is still able to configure the build with [`-DCMAKE_DISABLE_FIND_PACKAGE_Git=ON`](https://cmake.org/cmake/help/latest/variable/CMAKE_DISABLE_FIND_PACKAGE_PackageName.html) to disable git execution manually, for [reasons](https://github.com/bitcoin/bitcoin/pull/32220#issuecomment-2780115034) we don't know in advance.

  Closes https://github.com/bitcoin/bitcoin/issues/31999.

ACKs for top commit:
  maflcko:
    review ACK 3142e5f8cf 🥄
  fanquake:
    ACK 3142e5f8cf - tested a few different build scenarios (from Guix the tarball), they all seem to work as intended.

Tree-SHA512: 14995dc76fc680cff2bbc983d525125b39f49397e8ed94fffab29de505723a8e2f7f261c1a7ca88d98776755ccf63e59a32c476e112683f5cc1c016a7cfede94

cmake/script/GenerateBuildInfo.cmake

@@ -5,7 +5,7 @@
 macro(fatal_error)
   message(FATAL_ERROR "\n"
     "Usage:\n"
-    "  cmake -D BUILD_INFO_HEADER_PATH=<path> [-D SOURCE_DIR=<path>] -P ${CMAKE_CURRENT_LIST_FILE}\n"
+    "  cmake -D BUILD_INFO_HEADER_PATH=<path> -D GIT_EXECUTABLE=<path> [-D SOURCE_DIR=<path>] -P ${CMAKE_CURRENT_LIST_FILE}\n"
     "All specified paths must be absolute ones.\n"
   )
 endmacro()
@@ -28,72 +28,69 @@ else()
   set(WORKING_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 endif()
 
-set(GIT_TAG)
-set(GIT_COMMIT)
-if(NOT "$ENV{BITCOIN_GENBUILD_NO_GIT}" STREQUAL "1")
-  find_package(Git QUIET)
-  if(Git_FOUND)
+set(GIT_TAG "")
+set(GIT_COMMIT "")
+if(GIT_EXECUTABLE)
+  execute_process(
+    COMMAND ${GIT_EXECUTABLE} rev-parse --is-inside-work-tree
+    WORKING_DIRECTORY ${WORKING_DIR}
+    OUTPUT_VARIABLE IS_INSIDE_WORK_TREE
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    ERROR_QUIET
+  )
+  if(IS_INSIDE_WORK_TREE)
+    # Clean 'dirty' status of touched files that haven't been modified.
     execute_process(
-      COMMAND ${GIT_EXECUTABLE} rev-parse --is-inside-work-tree
+      COMMAND ${GIT_EXECUTABLE} diff
       WORKING_DIRECTORY ${WORKING_DIR}
-      OUTPUT_VARIABLE IS_INSIDE_WORK_TREE
+      OUTPUT_QUIET
+      ERROR_QUIET
+    )
+
+    execute_process(
+      COMMAND ${GIT_EXECUTABLE} describe --abbrev=0
+      WORKING_DIRECTORY ${WORKING_DIR}
+      OUTPUT_VARIABLE MOST_RECENT_TAG
       OUTPUT_STRIP_TRAILING_WHITESPACE
       ERROR_QUIET
     )
-    if(IS_INSIDE_WORK_TREE)
-      # Clean 'dirty' status of touched files that haven't been modified.
-      execute_process(
-        COMMAND ${GIT_EXECUTABLE} diff
-        WORKING_DIRECTORY ${WORKING_DIR}
-        OUTPUT_QUIET
-        ERROR_QUIET
-      )
 
+    execute_process(
+      COMMAND ${GIT_EXECUTABLE} rev-list -1 ${MOST_RECENT_TAG}
+      WORKING_DIRECTORY ${WORKING_DIR}
+      OUTPUT_VARIABLE MOST_RECENT_TAG_COMMIT
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      ERROR_QUIET
+    )
+
+    execute_process(
+      COMMAND ${GIT_EXECUTABLE} rev-parse HEAD
+      WORKING_DIRECTORY ${WORKING_DIR}
+      OUTPUT_VARIABLE HEAD_COMMIT
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      ERROR_QUIET
+    )
+
+    execute_process(
+      COMMAND ${GIT_EXECUTABLE} diff-index --quiet HEAD --
+      WORKING_DIRECTORY ${WORKING_DIR}
+      RESULT_VARIABLE IS_DIRTY
+    )
+
+    if(HEAD_COMMIT STREQUAL MOST_RECENT_TAG_COMMIT AND NOT IS_DIRTY)
+      # If latest commit is tagged and not dirty, then use the tag name.
+      set(GIT_TAG ${MOST_RECENT_TAG})
+    else()
+      # Otherwise, generate suffix from git, i.e. string like "0e0a5173fae3-dirty".
       execute_process(
-        COMMAND ${GIT_EXECUTABLE} describe --abbrev=0
+        COMMAND ${GIT_EXECUTABLE} rev-parse --short=12 HEAD
         WORKING_DIRECTORY ${WORKING_DIR}
-        OUTPUT_VARIABLE MOST_RECENT_TAG
+        OUTPUT_VARIABLE GIT_COMMIT
         OUTPUT_STRIP_TRAILING_WHITESPACE
         ERROR_QUIET
       )
-
-      execute_process(
-        COMMAND ${GIT_EXECUTABLE} rev-list -1 ${MOST_RECENT_TAG}
-        WORKING_DIRECTORY ${WORKING_DIR}
-        OUTPUT_VARIABLE MOST_RECENT_TAG_COMMIT
-        OUTPUT_STRIP_TRAILING_WHITESPACE
-        ERROR_QUIET
-      )
-
-      execute_process(
-        COMMAND ${GIT_EXECUTABLE} rev-parse HEAD
-        WORKING_DIRECTORY ${WORKING_DIR}
-        OUTPUT_VARIABLE HEAD_COMMIT
-        OUTPUT_STRIP_TRAILING_WHITESPACE
-        ERROR_QUIET
-      )
-
-      execute_process(
-        COMMAND ${GIT_EXECUTABLE} diff-index --quiet HEAD --
-        WORKING_DIRECTORY ${WORKING_DIR}
-        RESULT_VARIABLE IS_DIRTY
-      )
-
-      if(HEAD_COMMIT STREQUAL MOST_RECENT_TAG_COMMIT AND NOT IS_DIRTY)
-        # If latest commit is tagged and not dirty, then use the tag name.
-        set(GIT_TAG ${MOST_RECENT_TAG})
-      else()
-        # Otherwise, generate suffix from git, i.e. string like "0e0a5173fae3-dirty".
-        execute_process(
-          COMMAND ${GIT_EXECUTABLE} rev-parse --short=12 HEAD
-          WORKING_DIRECTORY ${WORKING_DIR}
-          OUTPUT_VARIABLE GIT_COMMIT
-          OUTPUT_STRIP_TRAILING_WHITESPACE
-          ERROR_QUIET
-        )
-        if(IS_DIRTY)
-          string(APPEND GIT_COMMIT "-dirty")
-        endif()
+      if(IS_DIRTY)
+        string(APPEND GIT_COMMIT "-dirty")
       endif()
     endif()
   endif()